Citrus should be fed starting in Spring and through Summer. Young trees can be fed once every three months during the cooler periods, but you should not feed mature trees at all during this dormant time.
Poultry manure is an excellent complete fertiliser that can be dug into growing beds and applied every six weeks during the growing period. For a less hands-on approach, consider a controlled-release citrus fertiliser applied once every six months or as per directions on the product. A controlled-release fertiliser releases a continuous and balanced supply of nutrients as required. The additional benefit of the controlled-release fertiliser is that they are typically formulated with trace elements essential for a healthy plant.
Apart from a regular yearly regime of fertilising your citrus, some indicators to look out for will let you know it’s time to feed. If the citrus looks healthy, its foliage is dark-green, and the fruit is fully ripening on the tree, it's likely to be adequately fed. If foliage is sparse and it’s of a lighter colour or yellowing, or you notice fruit dropping early, it’s time for a feed. Regardless of the appearance, it’s always best to feed your plants yearly, as this will prevent a lack of nutrients during the growing season. - MitchellMc
The likes of lemons and limes are really hungry at the start of spring. It's especially important for trees you have in pots and will set you up for plenty of fruit. - CathD